1. Technical Field
The present invention relates to an image sensor, more specifically, an apparatus and a method of smoothing the brightness of an image photographed by an image sensor without amplifying a noise component of a surrounding part of the image.
2. Description of the Related Art
An image sensor refers to the semiconductor device converting an optical image into an electric signal. Among this, a charge coupled device (CCD) refers to the device in which individual metal-oxide-silicon (MOS) capacitors are placed very close to each other and charge carriers are stored in and transmitted to the capacitors. However, a complementary MOS (CMOS) image sensor refers to the device employing a switching method, which makes as many MOS transistors as the number of pixels by using the CMOS technology using a control circuit and a signal processing circuit as a peripheral circuit and successively detects the output of the pixels by using the MOS transistors.
Portable apparatuses (e.g. digital cameras and mobile communication terminal) having image sensors are now developed and on sale. The image sensor consists of the arrays of small photo diodes, which are called pixel or photosite. The pixels themselves typically do not extract color from light. The pixels merely convert photos, provided from a wide spectrum band, into electrons. To write a color image by using a single sensor, a sensor is filtered such that different pixels can receive different color light. This type of sensor is well-known as a color filter array (CFA). The different color filters intersect the sensor and are arrayed in a predetermined pattern.
A color filter array of a color image typically is typically adjusted to the Bayer pattern. In other words, the half of total numbers of pixels is assigned to green G. Each quarter of the total numbers is assigned to red R and blue B. To get color information, color image pixels has a pattern repeated with red, green, and blue filters. For example, the Bayer pattern has a 2×2 array.
The Bayer pattern is based on the premise that a user's eye derives most of luminance data from the green component of an image. Accordingly, the RGB color filter alternated with more green pixels than other color pixels can generate a higher contrast image than the RGB color filter alternated with the same ratio of red, green and blue pixels.
The first one of elements directly related to the image quality of the image sensor is the lens concentrating light on the image sensor. The lens is required to have the properties such as good concentration of the focus point of light on the image sensor, penetration of more amount of light and uniform permeability of light into whole photographing surface.
A recently developed and purchased portable apparatus has the trends toward slim appearance and miniaturization, which mean all sensor modules become slim and compact. Accordingly, a corresponding image sensor equipped in the portable apparatus is required to have high resolution. As a result, enough distance is not acquired between a lens and a photographed surface. The brightness of the lens is not bright enough. The permeability of the lens is not uniform. In particular, the more distant the lens is toward an outside, the less the amount of light becomes.
FIG. 1 illustrates an image of an image sensor and an area thereof having different features, FIG. 2 illustrates features of an image per area and FIG. 3 illustrates a method of compensating features of an image per area.
Referring to FIG. 1, the feature of the image 100 is typically changed in the direction from a center pixel 110 of a center part thereof toward each edge pixel 120a, 120b, 120c and 120d (hereinafter, collectively referred to as 120). In other words, portions having similar features can be recognized by each concentric ring 130a, 130b, 130c and 130d. 
FIG. 2 shows the brightness, depending on the position of a pixel in the image 100, of various features. A first curve 210 indicates the maximum brightness depending on each pixel, and a second curve 220 indicates the minimum brightness depending on each pixel. The first curve 210 and the second curve 220 are brightest in the center pixel 110 and darkest in the edge pixel 120. The brightness of pixels get lower as the pixel position is changed from the center pixel to the edge pixel.
If a dynamic range D1 of the center pixel 110 is compared with dynamic ranges D1 and D2 of the edge pixels 120, the dynamic range D1 of the center pixel 110 is wider. Here, the dynamic range refers to the difference between the darkest brightness and the brightest brightness capable of being expressed in a corresponding pixel. In other words, the wide dynamic range leads to the high contrast, and the narrow dynamic range leads the low contrast.
If the dynamic range D1 of the center pixel 110 is compared with dynamic ranges D1 and D2 of the edge pixels 120, the difference occurs from 30 to 40% at the maximum depending on the lens feature of the image sensor. When it comes to the brightness, the surrounding parts having the edge parts 120 are easily affected by the noise relatively as compared with the center part having the center pixel 110. Accordingly, the compensation is needed.
For the compensation, referring to FIG. 3, the dynamic ranges of the whole image are attempted to be smoothed based on the dynamic range of the center pixel 110 (referring to a first arrow 310 and a second arrow 320). Accordingly, the dynamic range D2 of the surrounding part (having the edge pixel 120) is changed into D2′. For this, a gain of a certain rate is multiplied or a device performing a lens shading compensation function is used in order to compensate the dynamic ranges of the whole image. However, in this case, the noise component is amplified together in the surrounding part having the edge pixel 120, to thereby lower the contrast in the surrounding parts of the image 100 and deteriorate the quality of the image 100.